Former champion Wawrinka only returned from a knee injury in Melbourne and there was plenty of rustiness in his game against the appropriately named Tennys Sandgren, the American taking full advantage to continue his best grand slam performance.

Sandgren had never won a match at a major event before this week, but has now beaten a three-time grand slam winner and will meet Maximilian Marterer - who ousted Fernando Verdasco in five sets - in round three.

Wawrinka was not the only big name to depart on day four with Goffin and 2017 Wimbledon semi-finalist Sam Querrey also suffering surprise defeats to Benneteau and Marton Fucsovics respectively.

"I only had surgery five months ago, five months and three days," said a philosophical Wawrinka. "I think I'm way ahead of what I should be, that's for sure. That's why I need to be positive.

"Today was extremely tough, but I need to take what I can to be positive with everything, with the big picture."

DJOKOVIC AND MONFILS QUESTION "BRUTAL HEAT"

From the scoreline, Djokovic's victory over Monfils looked relatively straightforward, the Serbian coming from a set down to win 4-6 6-3 6-1 6-3 in just under three hours.

However, the numbers did not tell the full story as both players battled extreme heat in Melbourne, the mercury rising to 42 degrees Celsius at times on Rod Laver Arena, conditions they were critical of post-match.

"The conditions were brutal, that's for sure," Djokovic told a media conference. "I mean, we both struggled."

Monfils added: "It was tough to breathe. I think it was the hardest I have [played in]. I got super dizzy, I think I had small heat stroke for 40 minutes. I couldn't feel fresh. I tried to cool down, but even with the ice towel, the water [I could not]. I think my body was super warm."

Fifth seed Dominic Thiem would have been expected to brush aside qualifier Denis Kudla on Margaret Court Arena, but the Austrian found himself two sets down to the American after two hours on court.

Thiem was not to be denied, though, as he rallied to edge a five-set thriller 6-7 (6-8) 3-6 6-3 6-2 6-3, a feat he never gave up on despite his early problems.

"Of course, it's very tough [being two sets down]," he said. "Not easy mentally, but from when I was a break up in the third set, the mind was again positive. I always knew that if I play well and if I keep it up that I could turn it around."